1. Field of the Invention
The present invention relates to a lithium-ion secondary battery. More particularly, this relates to a lithium-ion secondary battery, comprising polymer particles as a binder showing lithium ion conductivity due to including an electrolyte, and having small internal resistance and excellent load characteristics, and low reduction in capacitance due to repeated discharge and charge.
2. Description of the Related Art
In recent years, portable terminals such as notebook-sized personal computers, mobile phones and PDAs (Personal Digital Assistant) show growing popularity. As electronic power supplies of the portable terminals, nonaqueous electrolyte secondary batteries such as lithium-ion secondary battery are heavily used. Also, nonaqueous electrolyte secondary batteries are recently attracted attention in using as large-scaled electronic power supplies for electric cars and for electric power storage because of environment concern and resources problem. For large-scaled electronic power supplies for electric cars, high load characteristics are required, and for electric power storage usage, high lifetime characteristics are required.
In the lithium-ion secondary battery, a binder is used for mutually binding electrode active materials, and binding an electrode active material to a collector. Since the binder is nonconductive and not ion-conducting, large usage has caused increase in internal resistance and reduction in capacitance. Synthetic rubber-based polymer particle-type binder is proposed to use as a binder with strong binding force even when using small amounts (Patent Document 1). Then, as the synthetic rubber-based polymer particle type binder, styrene-butadiene rubber polymer particle, nitrile-butadiene rubber polymer particle, methyl methacrylate-butadiene rubber polymer particle, etc., are disclosed. These synthetic rubber-based polymer particle type binders show improvement in internal resistance and capacitance decrease while load characteristics are not sufficient.
Also, a lithium polymer battery using polymeric (polymer) gel-like electrolytes containing an organic electrolyte has been developed as a new type of thin-and-light batteries. The polymer electrolyte battery has been subject to various studies since Armand, et al. proposed a battery using polymer electrolyte comprising polyethylene oxide and electrolyte salt (Patent Documents 2 and 3). However, when using these polymers for gel electrolytes as binders, it results in insufficient binding strength and lowered cycle characteristics as well as safety problem such as short circuit due to peeling of an active material layer.
Patent Document 1: The Japanese Unexamined Patent Publication 2004-55493,
Patent Document 2: The Japanese Unexamined Patent Publication 2000-173608, and
Patent Document 3: WO97/48106